ALMA 873 $μ$m Polarization Observations of the PDS~70 Disk

TL;DR

This paper presents deep ALMA 873\,μm full-polarization observations of the PDS 70 protoplanetary disk, a system with two confirmed accreting planets, to constrain dust grain growth via self-scattering. By combining Band 7 polarization with multi-band continuum data and performing Monte Carlo radiative transfer modeling, the authors infer a maximum dust grain size of roughly $a_{\rm max} \approx (70-90)\,\mu$m in the rings and crescents, and a marginally optically thick ring on scales $\gtrsim$10 AU. The polarization patterns, together with azimuthal variations in the spectral index $\alpha_{\rm b6-b7}$, support a fragmentation-limited growth scenario with fragmentation velocity $v_{\rm frag}$ of order $1\,\rm m\,s^{-1}$ outside the water snowline. This suggests that stickiness due to ice coatings is insufficient to grow grains beyond $\sim$100\,μm in these regions, highlighting fragmentation or bouncing as key barriers in the natal disk environment that forms planets, with future multi-frequency polarization offering a decisive test of the model.

Abstract

At a 112.4 pc distance, the PDS70 protoplanetary disk is a rare case that has been confirmed to host two accreting planets. This makes it the most important laboratory for studying dust growth in the context of planet formation. Here we present the first deep, full polarization observations at 873 $μ$m wavelength. We detected $\sim$1%-2.5% linear polarization over the bulk of the $\sim$55-100 AU (sub)millimeter ring. The polarization position angles align preferentially with the projected minor axis of the disk. The standard interpretation is that the observed polarization is caused by dust self-scattering, with a maximum dust grain size of $\sim$100 $μ$m. On $\gtrsim$10 AU scales, which can be resolved by the presented 873-3075 $μ$m observations, the ring is marginally optical thick at 873 $μ$m wavelength. Using Monte Carlo radiative transfer simulations, we found that an azimuthally asymmetric, marginally optically thick ring with a maximum dust grain size of $\sim$87 $μ$m can reproduce the observed 873 $μ$m polarization position angles and percentages. This study indicates that the coagulation of ice-coated dust in the protoplanetary disk may be limited by fragmentation or bouncing.

Figures (6)

• Figure 1: The 873 $\mu$m full polarization images on PDS 70. Left panel shows the total intensity (color scale) and polarization position angles (line segments). Right panel shows the total intensity (contours) and the polarization fraction. Contour levels are 197 $\mu$Jy beam$^{-1}$ (5-$\sigma$) $\times$[3, 6, 12]. The cross symbol marks the location of the host star. Filled and open circles mark the locations of PDS 70b and 70c, and their L4 and L5 points. Synthesized beam is shown in the lower left.
• Figure 2: Total intensity images at 1226, 1287, 2068, and 3075 $\mu$m wavelengths. Contours for the $\lambda=$1226 $\mu$m image are 81 $\mu$Jy beam$^{-1}$ (3-$\sigma$) $\times$[6, 12, 18]; contours for the $\lambda=$1287 $\mu$m image are 60 $\mu$Jy beam$^{-1}$ (3-$\sigma$) $\times$[5, 10, 15]; contours for the $\lambda=$2068 $\mu$m image are 36 $\mu$Jy beam$^{-1}$ (3-$\sigma$) $\times$[8,16,24]; contours for the $\lambda=$3075 $\mu$m image are 18 $\mu$Jy beam$^{-1}$ (3-$\sigma$) $\times$[4,8,12]. Symbols are the same as those in Figure \ref{['fig:b7pol']}. Synthesized beams are shown in the lower left. Yellow line segments indicate the deprojected position angles starting from 0$^{\circ}$ (defined at the major axis in the northwest), with 30$^{\circ}$ intervals.
• Figure 3: The 873--1226 $\mu$m spectral index ($\alpha_{\rm b6-b7}$). Color image in the left panel shows the spatially resolved distribution of $\alpha_{\rm b6-b7}$. Contours are the same as those in the right panel of Figure \ref{['fig:b7pol']}. The circles (with 30$^{\circ}$ intervals in deprojected position angles) indicate the 50, 60, 70, and 80 AU deprojected radii. Right panel shows the values of $\alpha_{\rm b6-b7}$ at 50, 60, 70, and 70 AU deprojected radii as functions of deprojected position angle $\phi$.
• Figure 4: Physical properties derived from the fiducial radiative transfer model. Left panel shows the synthesized, $\lambda=$873 $\mu$m images of total intensity (contours), polarization fraction (color), and polarization position angle (yellow line segments). We overplot the polarization position angles detected in actual observations (Figure \ref{['fig:b7pol']}). The angular resolution of these synthesized images are the same as those in Figure \ref{['fig:b7pol']}. Contour levels are 197 $\mu$Jy beam$^{-1}$ (5-$\sigma$) $\times$[3, 6, 12]. Synthesized beam is shown in the lower left. Right panel shows the fragmentation velocity ($v_{\rm frag}$) derived based on the maximum dust grain sizes ($a_{\rm max}$) in the PDS 70 rings.
• Figure 5: A comparison between the fiducial SED model (black dots; Methods) and observational data (colorful dots; Hashimoto2012ApJ...758L..19HLong2018ApJ...858..112LKeppler2019AA...625A.118KFacchini2021AJ....162...99FLiu2024ApJ...972..163L).